Prior-art techniques concerning the fluororesin-coated articles according to the first and second aspects of the present invention which will be described later, are first explained.
In conventional methods for adhering PTFE or the like to metals, the metal surface is roughened by blasting such as sandblasting, grid blasting, or the like, subsequently a fluororesin primer containing an adhesive such as
(poly(amideimide)), PES (polyethersulfone) or the like is applied on the surface, and then a fluororesin is coated thereon. However, such methods have the following problems:
(1) The fluororesin-coated metal plate cannot be subjected to so-called post-processing in which the coated plate is shaped by pressing into a pot, frying pan, etc., because the primer coating has too low an elongation at break. PA1 (2) White coatings or colored coatings such as those of bright colors cannot be formed since the PAI, PES or the like turns brown upon baking. Even if the coating may be colored gold, light brown or the like to utilize the brown color of the PAI, PES or the like, the color fastness of the coating is so poor that these colors fade upon exposure to sunlight, etc. Therefore, the coating color is practically limited to dark colors such as black and dark colors of metallic tones. PA1 (A) A fluororesin-coated article in which the heat-fusible fluororesin is PFA (a tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer); PA1 (B) A fluororesin-coated article in which the heat-fusible fluororesin is FEP (a tetrafluoroethylene-hexafluoropropylene copolymer); PA1 (C) A fluororesin-coated article in which the base plate is an aluminum or aluminum-alloy plate having a surface which has been finely roughened by chemical or electrochemical etching; and PA1 (D) A fluororesin-coated article in which the fluororesin contains substantially no adhesive other than the heat-fusible fluororesin. PA1 (A) A fluororesin-coated article in which the heat-fusible fluororesin is PFA; PA1 (B) A fluororesin-coated article in which the heat-fusible fluororesin is FEP; PA1 (C) A fluororesin-coated article in which the base plate is an aluminum or aluminum-alloy plate having a surface which has been finely roughened by chemical or electrochemical etching; PA1 (D) A fluororesin-coated article in which the inorganic pigment is at least one member selected from the group consisting of mica, pigment-coated mica, titanium oxide, fluorinated graphite, graphite and carbon; and PA1 (E) A fluororesin-coated article which further comprises a surface layer formed on the fluororesin composition layer, the surface layer consisting essentially of a fluororesin and containing substantially no pigment and adhesion aid. PA1 (A) A fluororesin-coated article in which the heat-fusible fluororesin is PFA; and PA1 (B) A fluororesin-coated article in which the heat-fusible fluororesin is FEP.
In another method for adhering fluororesins to metals, the metal surface is extremely finely roughened by chemical or electrochemical etching and a fluororesin coating is formed thereon, thereby bonding the coating to the metal by means of an anchoring effect. According to this method, the coating film can retain its elongation at break and have sufficient adhesion, so that the coated metal plate can be press-shaped in post-processing.
Even in the above method, however, if a pigment is incorporated into a fluororesin composition in order to obtain a colored fluororesin coating, adhesion to the metal surface is severely impaired and practically required bonding strengths cannot be obtained.
In addition, even in the case of incorporating no pigment, coatings utilizing an anchoring effect are caused to show significantly impaired adhesion to the metal surface and be apt to develop pinholes when the fluororesin composition is applied to metal plates which, after being coated, are subjected to post-processing under severe pressing conditions such that the thickness of aluminum base plate is reduced by more than 10% of the original thickness. Because of this, it has been difficult to obtain coated articles having good corrosion resistance.
In still another method which is a further improvement of the above technique, an adhesive such as PAI, PES, or the like is incorporated into a fluororesin composition along with a pigment in such amounts as to not severely impair the elongation at break of the coating, subsequently the composition is coated on the finely roughened surface of a base plate, and then a fluororesin layer containing substantially no pigment and adhesive is formed as all outermost layer, whereby a coated article capable of being subjected to press shaping and exhibiting a minimized deterioration in surface non-tackiness due to the incorporation of adhesive can be obtained.
However, the above improved method is still insufficient in producing highly corrosion-resistant coated articles under severe pressing conditions because the incorporation of PAI, PES, or the like tends to decrease the elongation at break of the coating and increase the number of pinholes formed in the coating.
Further, since the PAI and PES turn brown upon baking, the problem that white coatings turn beige and coatings of bright colors come to be of dim colors remains unsolved. Furthermore, since these colors fade upon exposure to sunlight, etc., only coatings of dark colors, metallic-tone dark colors and the like are obtained. Although it has also been attempted to obtain white or bright colors by utilizing a color fading phenomenon due to sunlight, etc., i.e., by decoloring the brown color of PAI or PES after baking by means of an ultraviolet lamp, this technique is disadvantageous in cost.
Prior-art techniques concerning the fluororesin-coated article according to the third aspect of the present invention which will be described later, are now explained.
In order to obtain a highly corrosion-resistant fluororesin-coated article such as a PTFE-coated article, it is necessary that the coating should be exceedingly good in either (1) being free from coating defects such as pinholes or (2) having enhanced adhesion to the base plate, or should (3) have well-balanced properties with respect to the above (1) and (2).
It is normally extremely difficult to obtain coating films of PTFE which are completely free from pinholes and other defects. Therefore, coated articles exceedingly good in property (1) above have been obtained by a method in which a plurality of layers are formed by repeated coating operations to eliminate through-holes extending to the base plate. This method has been effective in considerably reducing the number of pinholes. However, the above method has a problem that as a result of an increased number of coating operations, the resin layers formed by earlier coating are exposed repeatedly to temperatures not lower than the sintering temperature to thereby deteriorate gradually. In addition, the above method has been disadvantageous in cost because of an increased resin amount and an increased number of steps.
On the other hand, coated articles excellent in property (2) above have been obtained by a method in which a fluororesin primer containing an adhesive such as PAI, PES or the like is applied on a base plate and a fluororesin layer is formed thereon. There is, however, a problem that such a primer layer is extremely prone to develop coating defects such as pinholes. Moreover, the coated articles produced by the above method have another problem in that their applications are limited since they cannot be subjected to so-called post-processing in which they are shaped by pressing into a pot, frying pan, etc., because of the low elongation at break of the coatings.
As an expedient for improving the above methods to attain (3), i.e., well-balanced properties of (1) and (2), a method has been attempted in which the surface of a metal plate is extremely finely roughened by chemical or electrochemical etching, a layer of a fluororesin with which a small amount of a primer has been mixed is formed on the roughened surface, thereby establishing adhesion based on a balance between an anchoring effect and adhesion by the primer, and then a fluororesin containing substantially no pigment and adhesive is coated thereon. Although coated articles produced by this method are considerably improved in practically required properties as compared with coated articles produced by the two methods described above, they are still insufficient for use in post-processing conducted under more severe conditions or for use as a pan for boiling, a grill pan for making sukiyaki, hotchpotch, etc., or the like which are used under more severe conditions than rice cooker-warmers.